Suppress clippy unused_self pedantic lint

This usage is fine. It's mirroring trait signatures in syn::visit::Visit.

    error: unused `self` argument
       --> serde_derive/src/bound.rs:241:24
        |
    241 |         fn visit_macro(&mut self, _mac: &'ast syn::Macro) {}
        |                        ^^^^^^^^^
        |
    note: the lint level is defined here
       --> serde_derive/src/lib.rs:18:22
        |
    18  | #![deny(clippy::all, clippy::pedantic)]
        |                      ^^^^^^^^^^^^^^^^
        = note: `#[deny(clippy::unused_self)]` implied by `#[deny(clippy::pedantic)]`
        = help: consider refactoring to a associated function
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_self

serde_derive/src/lib.rs

@@ -48,6 +48,7 @@
     clippy::struct_excessive_bools,
     clippy::too_many_lines,
     clippy::unseparated_literal_suffix,
+    clippy::unused_self,
     clippy::use_self,
     clippy::wildcard_imports
 )]